extrusion

Experiment 1

Extrusion

Date of Experiment: 03/12/2014

Technical Adviser: Sven Klees

Group name: 6

Student Name: Jiahao Xu

IGS M. Sc. Chemistry WS 2014/2015

Introduction

1.extruder

Extrusion is a continuous production process of Plastic Engineering for final processing. In extrusion (extruder from lat. = Float out) are pressed (in the case in heated thermoplastics) using heat and pressure continuously through a nozzle viscous and curable thermosetting materials, creating a body defined length arises. The nozzle determines here the design. The scope of this processing technique is diversified by the flexible, multi-pagein the industry. A high output, high-quality processing capability and a high degree of reproducibility, the extrusion is carried especially in the plastic and food industry. In the history, it was first used in the rubber industry.

An extrusion machine is composed of a screw shaft motor and thrust bearings, a dosage and cooling or heating device, a feeding and a cutting device. Furthermore, controls and a power supply for the electronic control are used: Prior to the extrusion itself, red layout dye was painted on the inside split of the billets and a height gauge was used to scratch lines every 0.100 inches both along the axis of the split billets, and perpendicular to the axis. This would allow a visualization of the flow characteristics after the extrusion.

Figure 1: Schematic structure of an extruder

The extruder produced a homogenous melt, so that the processed material can pass through the nozzle with the optimum temperature and pressure.

There are four distinct types of extrusion:

1. Single-screw extruders with a smooth feed zone and three-zone screw

2. Single-screw extruder with groove-feed zone and screw with mixing and shearing zone

3. Co-rotating twin-screw extruder

4. Counter-rotating twin-screw extruders

The type of nozzle determines the shape of the resulting product. Through the use of various types as fixed profiles (such as square tubes), hollow profiles (such as window frames), open profiles (such as channels), tubes, films and sheets can be produced.

For some special applications such as leaves, blown or products from different positions there are special extruder structures that are neglected in the following.

2.The process of extrusion

In the experiment, a single screw extruder with three screw zone and smooth feeding zone was used . This represents the conventional type of machine. This works follows the following schematic sequence:

About feeding the device is supplied with the raw material (polymer granulate). The cooling provides for preventing premature melt by the friction, since this makes it difficult to transport the granulate through the feed zone to the plasticizing zone by adhesion effects. In the plasticizing zone, the temperature may be increased by the friction. This zone is also called compression zone. The first material begins to melt on the cylinder wall, since it is heated. From a discrete viscosity of the cooled film on the front edge of the screw begins to liquefy and begins by adhesion to the screw to flow. This captures another unmelted material and causing it to melt. Through the "leakage flow" through the cylinder spaces more melts are initiated. This effect produces a transition zone. These fixed shares, however, are difficult to avoid completely, as poor heat transfer in the material takes place. In addition, the pressure gradient forms along the "leakage flows" a melt without adhesion. This melt is homogenized and separated, creating a melting priority emerges. However, this effect alone forms an insufficient dispersion of the solid granules in the melt to explain a completely homogeneous melt.

The inhomogeneous mixture having a temperature gradient is mixed here means of the flat incision and ejected through the nozzle with a defined temperature uniform. For some materials will be added to the described areas additional zones of mixing and separation to produce a qualitative and quantitative output as high as possible, so that a high efficiency is generated.

All these ingredients are subject to the requirement of a constant uniform material transport, a homogeneous melt with respect to temperature and material mixture and a decomposition outdoor treatment of the material.

Figure 2 Schematic representation of a 3-zone screw.

3. The material flow process

The transport of the material is done by the adhesion of the melt to screw and cylinder wall and passes through Coulomb friction. This flow of melt along the transport direction is called "drag-flow" . This friction increases the temperature of the material during transport, thus resulting in an inhomogeneous melt. Consequently, the mechanism of the transport incorrectly, which often cooling is used. By the pressure differential within the transport cylinder, there is a "pressure-flow" Gp, wherein in the extruder model used without groove has a negative effect on transportation. The material flow through the gap of the cylinder wall and screw "leakage flow" called Eq. This ensures a strong Scheer effect in the material flow.

The "drag-flow" Gs, by a material-specific constant Ks, the screw tread depth h and the rotational speed of the screw n can be expressed as follows:

Gs = K ∙ h ∙ n

The constant K is a quadratic function of the diameter D and the two angles changes from:

Ks=D²∙sinα∙cosα

The resulting proportionality shows that the "drag-flow" depends on the profile depth of the screw and the diameter squared:

GsH∙n∙D²

In order to increase the propulsion and the "drag-flow", it is particularly effective to increase the diameter of the screw. Especially since the increase in tread depth complicates uniform heating, since the volume of the heated surface increases.

The resulting material flows overlap and result in the total flux G:

G = Gs- Gp - Gl

4. Comparison of Smooth and Grooved Feed extruder

It is clear that the introduction of axial grooves formed in the cylinder wall with smoother velocity and thus a lower reflux by a low pressure gradient in the material. This is done by preventing the movement solidified material with the screw rotation. The friction is increased, and improves the transport properties of the material. The pressure on the melt feeding direction is minimized, while the pressure along the promotion direction is maximized. This has an additive effect of the pressure flow to episode, thus increasing the overall flow. The reduced emissions of conventional extruder by the dynamic pressure decreases the efficiency dramatically. However, doing the mixing is weaker than that of a conventional extruder and must be collected by the addition of a mixing zone, so that a homogeneous melt is ensured.

5.Thermoplastic processing

Using an extruder can processe structural polymers. Especially thermoplastic polymers such as PVC, PE, PP, PET and PA can be processed optimally through their agent properties. In this experiment polyethylene was used.

These consist largely of linear polymer strands. Among themselves , the individual chains interact by van der Waals forces and other vulnerable attractions. By adding heat to soften the thermoplastic materials that can be deformed because the linear unbranched polymer chains are now mutually movable.

This is due to the heating of the thermoplastic material above the glass transition temperature TG. This is represents a transition of the amorphous material into a rubbery highly viscous melt. This effect is reversible by cooling. the cooling was done too quickly, this compressive stresses are frozen because the chains are not longer fully portable. This leads to a reduction in quality and possible cracks in the material.

The cooling reduces the volume of the hot material. Also, a volume contraction is observed due to the applied pressure nozzles. The polymer chains are closely pressed together. If the pressure is at a suitable temperature, the polymer mixture can relax again and reaches the initial state, Above the glass temperature, so the origin of orientation can be produced (by homogenizing and mixing) again, as the entangled polymer strands without acting influences of pressure their arrangement can take. This effect is called Memory' effect.

When extruieren melt, fracture can occur in the material. This is a knot-like flow of the material to the nozzle. This is done by the pressure and the resulting expansion tension at the nozzle. When exceeding a critical flow breaks the melt and a visco-elastic rebound occurs, whereby the typical shape of the node extruierten mass can be explained. At constant screw speed it is a periodic sequence. The occurrence of melt fracture is thus the pressure and the temperature. At higher temperatures, the material is not as viscous and can be discharged at higher speeds.

6. implementation

The extruder was turned on and filled with PE resin. The amount of feeding was kept as constant as possible during the measurement. The extruder was run until the temperature and screw speed constancy before the measurement was started.

Choose proper times to get accrate values (45s) and determined at 30, 50, 70, 90 and 110 min-1 and mwasure the throughput.

Table 1: Temperature settings on the extruder for mass determination at T1 and T2

Then the cooling process of extruierten material was tested under different circumstances. For this purpose, a triple measurement of the filament diameter was performed. The produced short samples were cooled in air, in an ice bath and after a hot oil bath at 170°C.Notice it was taken to a horizontal cooling, to avoid deflection by the force of gravity. Thus, the sample was extruiert directly on a horizontal surface. In the oil and ice baths we should notic the smallest possible distance to the nozzle so that the air contact is very small.

observation resullt

The value of ejected mass under different revolution are showed in follow table 2 :

Table 2: Determination of the throughput masses

The flow rate is plotted against revolutions:

Table 3: Values Table for graphical plotting

X Axis is U /min Y Axis is m Kg /h

Figure 3: Plot diagram of m / U (working diagram)

The diameters of the samples under various cooling processes:

Table 4: Values of the diameter for cooling processes

Using the equation for swelling factor S = Ds / Dw and percentage S= Ds / Dw -1，caculate the percentage yields:

Dw = 1,75mm

Table 6: Swelling factor of the cooling processes

Conclusion

The first part of the experiment shows that the increase in temperature causes only a marginal difference in the ejected masses. It is, however, contrary to expectations even less mass ejected as at higher temperatures. This could be due to the lower viscosity of the higher-Tempered PE. This affects in this conventional model with higher pressure slightly higher than the low "drag-flow". For this reason, the almost identical line was showed in the work diagram. So here the straight rather than an expected exponential increase was showed.

These facts can be due to the pressure was only expressed in the form of screw rotational speed n, either through age-related inaccuracies of the extruder as well as to the lack of the exact pressure. Possibly the exponential relation is ignored. The second Experimentarteil concerned with determining the swelling factor and its percentage. Here it can be seen that the diameter of the cooled ice water in the sample is lower. It can be assumed that the compressive stress of the nozzle was frozen immediately, so that the PE could not relax. The ejected product in oil samples have a higher diameter. This is consistent with expectations.

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